Understanding Measurement Uncertainties In Spectrum Analysis

What is in this article?:

Understanding the codependent relationship between measurement accuracy and other parameters is crucial.

In previous articles, I have provided information on the relationship between measurement speed, repeatability, and dynamic range. In many cases, improving one of these parameters can be at the expense of the other two. An example of this would be performing less averaging which improves speed but has an adverse effect on repeatability and potentially can limit dynamic range. This article will detail the accuracy of measurements and a similar codependence this has with dynamic range, measurement speed, and repeatability.

The Specifications Guide

Any study on spectrum analyzers should begin with a look at published specifications for spectrum analyzers provided by their manufacturers. The Agilent N9020A MXA signal analyzer will serve as an example for this article. The instrument’s Specification Guide can be downloaded here.

It may be surprising that this specifications guide is nearly 400 pages in length. Agilent and other signal analyzer manufacturers spend considerable time deriving these specifications and then testing them in production and during instrument calibration to guarantee the performance of the instrument; should it not meet any of the guaranteed specifications, the instrument needs to be repaired. This adds an extreme amount of value to a test engineer who normally works within a measurement uncertainty, dynamic range, and test time budget and can now analytically determine what signal analyzer price-performance mix will work best in their test system today, as well as when additional signal analyzers may be needed when the production line or lab expands in the future.

Understanding a Specification Guide

The N9020A MXA signal analyzer specification guide is split into three columns. The first column is a description of the specifications as well as the range and parameters for which each specification applies. This can be the temperature range, such as 0 to +55°C in field-deployable applications, or +20 to +30°C for manufacturing or bench top use. The center column displays the guaranteed specifications. These specifications include the measurement uncertainties for the equipment used during production, as well as those used during calibration. Environmental guard bands are also included in these specifications, such as temperature range and humidity. Many users of Agilent test equipment see actual performance levels well beyond the specified performance limits. To address this, a third column provides supplemental information that might be useful to the test engineer. This might be typical performance beyond the specification that 80% of the instruments exhibit with a 95% confidence level over the temperature range +20 to +30°C that a user is most likely to observe. An example of a specification table is provided below.

Dynamic Range

The Specification Guide also provides performance values for various dynamic-range specifications, such as third-order intercept (TOI), phase noise, harmonics, and spurious response. These specifications, when used in conjunction with the accuracy specifications, should be used to determine what test limit is needed to guarantee that the performance of the device under test (DUT) meets the specification given for the DUT. Insufficient dynamic range or accuracy or a combination of these can be the source of DUT yield issues which can have significant implications in the cost of test. This can be the result of false failures in production or, even worse, the shipment of failed devices that supposedly passed.